Many test systems require periodic calibration to insure that the measurements made by the test system are accurate. Although system calibration can significantly reduce systemic or “bias” errors, other uncertainties arise that can not be calibrated out, and arise in a somewhat random fashion when testing electronic devices. In high-frequency test systems, measurement uncertainties often arise from impedance mismatches between components of the test system and from other sources, such as temperature stability, aging, noise, and accuracy and repeatability of the test instruments in the test system. Uncertainties from multiple sources can add to or subtract from each other to create a total measurement uncertainty for a measurement made by the test system.
In many cases, the measurement uncertainty of a test system or instrument is required to be reported along with the measured calibration and/or test values. Standards have been developed for calculating and reporting measurement uncertainties. The International Organization for Standards (“ISO”) provides specifications (e.g. ISO 17025) for making measurements and reporting uncertainties in a particular fashion. Therefore, knowing the uncertainty of a measured value is very important, and sometimes required.
Calibration of high-frequency electronic test equipment and test systems is often done automatically or semi-automatically. Computer software controls the electronic test equipment and can include mathematical equations that calculate the associated measurement uncertainty (“uncertainty calculations”) to provide the test result and associated measurement uncertainty in the desired format. These uncertainty calculations are conventionally derived by a highly trained metrologist who examines the specific test system being used and develops an uncertainty calculation based on the instrumentation and topography of the test system. This process can take several days.
If a test instrument in the test system is replaced, the values of uncertainties may change, but the form of the uncertainty calculations often do not. However, if the topography of the test system is changed, such as by adding or removing a switch in the signal path, then the form of the uncertainty equations changes and the metrological analysis must be re-validated by the metrologist, and the uncertainty calculation incorporated into the computer test software also must be changed.
Therefore, it is desirable to calculate measurement uncertainties for high-frequency test systems and instrumentation without manual metrological calculations.